Retiring Windows chief Jim Allchin announced the release Friday in an e-mail to testers. In his note, which was also posted to the Windows Vista Team blog, Allchin said there are "a lot of improvements since Beta 2," which was released in May. Among the changes he highlighted are tweaks in the user interface, more device drivers and improved performance.

Work on Vista is not done, Allchin wrote. "We'll keep plugging away on application compatibility, as well as fit and finish," he wrote. He noted that software makers should use the RC1 release to certify their applications.

The software maker is shooting to wrap up development work in time to ship the operating system to large companies in November and have a mainstream launch of Vista in January.

The release candidate is available to some technical beta and corporate testers now and will be made available next week to testers that are part of Microsoft's MSDN developer network. The software released Friday is build 5600, a slight update to the test version Microsoft released in the past week to businesses that are part of an early adopter program as well as to a subset of the tech enthusiasts that have been putting Vista through its paces.

Microsoft declined to say when the consumers that have been testing Vista will get their hands on RC1. The last broadly available test version, Beta 2, was released in May.